Impact of Fire on Herbivores

Fire plays an important role in the ecology and evolution of savanna and grassland ecosystems and is often used as a tool for the management of grassland vegetation. Mammalian herbivores have been shown to be attracted to recently burned areas. This attraction has largely been attributed to increases in plant nutrient content. However, because burning causes a reduction in vegetation height and increases in sighting distances, these areas may also be safer habitats from predators. To test these two different hypotheses we measured herbivore abundances as well as vegetation traits (height, biomass, and plant nutrient content) on unburned and newly burned areas in Serengeti National Park, Tanzania. The results of this study showed that generally smaller herbivores preferred burned areas more strongly than larger herbivores. Additionally, burning caused increases in leaf N and leaf non-N nutrients (Cu, K, and Mg), and a decrease in vegetation height and live:dead biomass. The increase in non-N nutrients was the main factor explaining herbivore preference for burned areas. However, the preference for burned areas and positive changes in vegetation characteristics is short term, lasting less than four months after burning occurs. Despite these short term increases in nutrient quality, increases in fire frequency (the number of times a site burns) can cause changes in plant species composition towards species of lower nutrient quality. 

Impact of Fire on Carnivores

Little is known about how fire affects short-term carnivore distributions across the landscape. Through the use of a long-term data set collected by the Serengeti Lion Project we investigated the distribution of lions in relation to burned areas in Serengeti National Park.  We found that lions avoid burned areas despite the increases in prey availability in these areas. This avoidance may be due to the above mentioned reduction in vegetation height seen in burned areas which may cause decreased hunting success.

Impact of Fire and Grazing on Plants

Historically, fire played a large role in savanna and grassland ecosystems, as both lightning and human set fires frequently occurred. Presently, humans are altering fire regimes by increasing or reducing the frequency of fires or completely excluding fires. Grazing also played a large role, historically, in these ecosystems, as many were home to large numbers of native megafauna. Humans are altering grazing by completely removing native grazers or replacing them with livestock. It is important to have a more general understanding of the impacts of these drivers on these ecosystems in order to predict how they will behave with continued human influence.

In order to study the impacts of these drivers, sites were established in Kruger National Park, South Africa and at the Konza prairie long-term ecological research site in Kansas, USA. In order to test the impacts of fire and grazing on plant species composition (richness, diversity, and evenness) exclosures and corresponding grazed paired plots were established on fire treatments that burn annually or never burn.

There is a large difference in herbivore diversity between the two sites (one species at Konza and 14 at Kruger). In an attempt to remove any effect that the greater herbivore diversity at Kruger might have, sites there were established inside a buffalo enclosure. This allowed us to compare the impact of a single grazer at Kruger (Cape buffalo, Syncerus caffer) to a single grazer at Konza (Bison, Bos bison). We found a decrease in grass and forb richness, evenness and diversity over time when a single grazer was removed from each site. However, in Kruger these changes only occurred with burning. At both sites these changes occurred because of increases in the abundance of dominant grass species within exclosures.